Back in 2009, my remodeling company took on the renovation of a 105-year-old three-story brick structure, vacant for the past 10 years, here in Louisville, Ky. The job included an extensive interior gut and energy rehab along with a new, one-story addition at the back. The original standing-seam terne metal roof (terne is a lead/tin alloy) was intact but in need of attention. We wanted to both restore the original roof and match its appearance on the roof of the addition. As with most remodeling jobs, the budget was a driving concern. But we also wanted to make an example of this home, to show how builders can conserve existing materials when possible.

Instead of Reroofing, Waterproofing

While it may have been more expedient to tear off and replace the old roof, it would also have been costly and unnecessarily wasteful. Instead, we decided to revive the old, corroded metal with a product called Hydrostop PremiumCoat (800/739-5566, www.hydro-stop.com), an acrylic elastomeric liquid membrane that's applied over a reinforcing polyester fabric. The product is specifically designed for roofing; it's self-flashing and relatively easy to work with. Rather than do everything ourselves, however, we opted to work with a certified installer in order to secure the 10-year warranty. If certified maintenance is performed every decade, the warranty can be extended more or less indefinitely.

Hydrostop can also be used to waterproof metal, masonry, and plywood. This meant that not only could we restore the old metal roof, but by applying "dummy" standing seams to the plywood substrate on the new roof of the addition, we could use the liquid membrane to create a convincing imitation of the old roof.

An added benefit - though not required by any code or regulation - was that the new coating would encapsulate the terne's high lead content, preventing it from future leaching.

Hydrostop fully cures in about seven days but dries to the touch on the day of application. It comes in 16 standard colors, with custom colors available at extra cost. Once cured, it can also be painted.

The installed cost for coating the existing metal roof came to $8.10 per square foot, which was roughly half the cost of standing-seam copper roofing and close to the cost of new 26-gauge Galvalume - but only if you don't factor in the cost to remove and dispose of the old roofing. All things considered, Hydrostop was our least expensive option.

Prepping the Existing Metal Roof

Some years earlier, a coat of rust-red paint - which was now peeling - had been applied to slow corrosion. To prepare the metal for the new coating, the installers first cleaned it with a pressure washer and a wire brush to remove loose paint and other particles that might interfere with a good bond. Past "repairs" included layers of roofing tar applied over chimney and roof vent flashings, but this was not a great concern because Hydrostop effectively bonds to dried asphalt and old asphalt roofing. As long as the material in question is firmly secured to the roof and adjacent surfaces, it can be successfully coated.

After allowing the roof to fully dry, the installers applied a rusty-metal primer. Its green color helped to highlight any inadequately treated areas; these were recleaned and primed before proceeding. We removed a couple of abandoned plumbing vents along with two of three ridge-top attic vents that were rendered obsolete in the remodel. (The remaining vent - which serves a bathroom fan - was simply coated along with the roofing.)

We screwed a piece of painted aluminum flashing over the old vent holes. The paint on the aluminum and the primer on the roof will prevent any corrosive reaction between the two metals. Under the thick finished application, these patches are nearly invisible. The same holds true for the new vent flashings, which we fastened to the surface of the primed metal roofing and embedded in the subsequent coating layers. One of the beauties of Hydrostop is its repairability - we could have easily added these vents at a later time while still maintaining the monolithic character of the coating.